Mermaid vs PlantUML for Sequence Diagrams
Both Mermaid and PlantUML let you write sequence diagrams as plain text. Neither is objectively better — the right one depends entirely on where the diagram needs to live and who needs to read it.
The short version
- Use Mermaid if the diagram lives in a GitHub or GitLab repo, Notion, or Obsidian
- Use PlantUML if your team uses Confluence or JetBrains IDEs
- Use an online editor if you need to share fast, export an image, or convert between formats without any toolchain
Same diagram in both syntaxes
Here is the same OAuth login flow written in Mermaid and PlantUML side by side, so you can compare the syntax directly:
sequenceDiagram
actor User
participant Browser
participant "Auth API" as API
participant Database
User->>Browser: Enter credentials
Browser->>API: POST /auth/login
API->>Database: Find user
Database-->>API: User record
API-->>Browser: 200 OK + JWT
Browser-->>User: Redirect to dashboard@startuml
actor User
participant Browser
participant "Auth API" as API
participant Database
User -> Browser : Enter credentials
Browser -> API : POST /auth/login
API -> Database : Find user
Database --> API : User record
API --> Browser : 200 OK + JWT
Browser --> User : Redirect to dashboard
@endumlThe differences are mostly punctuation. Mermaid uses ->> for solid arrows; PlantUML uses ->. PlantUML requires @startuml / @enduml wrappers and puts colons after the target (API : message) rather than before (API: message).
Where each one renders natively
This is the most important practical difference. "Renders natively" means you paste the syntax and it just works — no plugins, no CI steps, no external servers.
| Platform | Mermaid | PlantUML |
|---|---|---|
| GitHub (markdown, PRs, wikis) | Native | Requires CI / third-party |
| GitLab (markdown, MRs, wikis) | Native | Not native |
| Notion | Native (code block) | Not native |
| Obsidian | Native (code block) | Via plugin |
| Confluence | Requires plugin | Native (PlantUML macro) |
| IntelliJ IDEA / JetBrains | Via plugin | Native (PlantUML Integration) |
| VS Code | Via extension | Via extension |
| Doxygen | Not native | Native (directive in comments) |
Toolchain requirements
Mermaid
No local install is needed if your platform renders it natively (GitHub, GitLab, Notion). For local rendering, the Mermaid CLI requires Node.js. There is no server-side component — the JavaScript library runs in the browser.
PlantUML
Local rendering requires a Java runtime. The PlantUML .jar is run on the command line to export SVG or PNG. The hosted renderer at plantuml.com sends your diagram to an external server, which may be a concern for private system architecture diagrams. JetBrains IDEs bundle the renderer in their PlantUML plugin, so no separate Java setup is needed there.
Syntax differences that matter
| Feature | Mermaid | PlantUML |
|---|---|---|
| Solid arrow | A->>B: msg | A -> B : msg |
| Dashed arrow | A-->>B: msg | A --> B : msg |
| Activation box | activate A | activate A |
| Loop fragment | loop condition | loop condition |
| Alt/else fragment | alt / else | alt / else |
| Note | note over A,B: text | note over A,B : text |
| Required wrapper | sequenceDiagram (optional in many tools) | @startuml / @enduml required |
Version control
Both formats are plain text, so both work well in git. Diffs are readable, merges are possible, and the source file documents the flow alongside the code that implements it. This is one of the main reasons to prefer either over a drag-and-drop diagramming tool.
The meaningful difference is file extension convention: .mmd for Mermaid, .puml or .plantuml for PlantUML. Both are text files.
How to decide
Pick based on where the diagram needs to live
If your team uses both platforms, pick the format that matches the canonical destination and convert to the other when needed. SeqDiagram can import either format and export to the other in two steps — no local tooling required.
Frequently asked questions
Should I use Mermaid or PlantUML for sequence diagrams?
Use Mermaid if your diagrams live in GitHub, GitLab, Notion, or Obsidian — it renders natively in all of them. Use PlantUML if your team works in Confluence or JetBrains IDEs, where PlantUML renders natively. If you need to share fast without any setup, an online editor like SeqDiagram works for both and lets you convert between formats.
Can I convert between Mermaid and PlantUML?
Yes. In SeqDiagram, import a Mermaid diagram via Import Mermaid and export to PlantUML, or import a PlantUML diagram via Import PlantUML and export to Mermaid. No local tooling needed.
Does Mermaid work in Confluence?
Not natively. Confluence renders PlantUML natively via the PlantUML macro. Mermaid in Confluence requires a third-party plugin. If Confluence is your primary documentation platform, PlantUML is the lower-friction choice.
Does PlantUML work in GitHub?
Not natively. GitHub renders Mermaid natively in markdown files and PR descriptions. PlantUML in GitHub requires a CI step or a third-party renderer. If GitHub is your primary platform, Mermaid is the lower-friction choice.